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Tuesday, December 30, 2014

Janis Weis' group has been mapping genetic variants that make laboratory mice prone to severe Lyme arthritis. One of these variants is described in a paper that appeared in The Journal of Clinical Investigation earlier this year. The affected gene encodes the enzyme β-glucuronidase, which carries out a critical function in the lysosome. β-glucuronidase cooperates with other degradative enzymes in the lysosome to break down glycosaminoglycans (GAGs) into their individual sugar units, which are then removed from the lysosome and reused by the cells. GAGs are long chains of specific disaccharides located on the cell surface and within the extracellular matrix. GAGs are covalently (in proteoglycans) or noncovalently attached to proteins. GAGs are always being degraded and resynthesized by cells. Blocking any of the enzymes involved in GAG breakdown causes accumulation of GAG fragments, which are potentially detrimental to health. In humans, certain mutations in the β-glucuronidase gene lead to a rare condition called Sly syndrome.

Large amounts of GAGs are found in the joints, where they serve an important mechanical function. GAGs carry a high density of negative charge due to the presence of acidic sugars such as glucuronic acid, the target of β-glucuronidase, and the sulfate groups attached to most types of GAGs. The negative charge attracts cations, which in turn
attract large numbers of water molecules. The water within GAGs acts as a cushion that allows
the joints to withstand large compressive forces.

The key to the study was having strains of mice that differed in their susceptibility to Lyme arthritis. The C3H mouse strain develops severe joint inflammation during B. burgdorferi infection. On the other hand, the B6 strain develops mild joint inflammation when infected. Weis' group had earlier narrowed the locations of the genetic variations accounting for the different susceptibilities to several distinct segments within the mouse genome. They used the traditional techniques of mouse genetics, which involved numerous matings involving the C3H and B6 strains and their progeny (see this review for details). The authors focused on one of the segments, and with help from mouse genome sequence data that became available, they found a nucleotide difference within the Gusb (β-glucuronidase) gene that changed a single amino acid in the enzyme.

The investigators found that β-glucuronidase activity was mildly reduced in the infected C3H strain relative to the B6 strain. Staining tissue sections of infected mice with Alcian blue, a dye attracted to polyanions, revealed accumulation of GAGs in the joint tissues of infected C3H mice but not infected B6 mice, lending further support to the lesion in Gusb being responsible for severe inflammation. When a functional copy of the β-glucuronidase gene was stitched into the genome of C3H mice, B. burgdorferi infection no longer caused joint inflammation.

Does the same process occur in humans with Lyme
arthritis? One hint that β-glucuronidase influences the course of Lyme
arthritis is the finding from other labs that found that the concentration of the enzyme in joint fluid is higher in patients with Lyme arthritis than it is in healthy
uninfected individuals, although how the high enzyme levels are mechanistically linked
to arthritis remains unexplained.

So how does β-glucuronidase deficiency lead to severe Lyme arthritis? One possibility raised by the authors is that GAG fragments worsen tissue inflammation by stimulating Toll-like receptors, as shown in other studies (see this paper for an example).

The findings may also tell us something about rheumatoid arthritis (RA). The B6 strain ends up with a form of RA following injection with certain autoantibodies. One of Weis' mouse crosses generated a B6 strain with its Gusb gene and flanking regions swapped for the same region of the C3H strain. This strain developed severe arthritis when injected with the same autoantibodies and when infected with B. burgdorferi. Therefore, the pathologic processes leading to Lyme arthritis and RA share common steps, at least in laboratory mice. In humans, RA patients, like those with Lyme arthritis, have high levels of β-glucuronidase levels in their joint fluid.

Feeding Ixodes ticks harboring Borrelia burgdorferi deposit the Lyme disease spirochete in the skin of the victim. The spirochetes remain...

Common Spirochete Diseases

Lyme disease is a tick-borne disease caused by several members of the Borrelia burgdorferi complex. B. burgdorferi, B. garinii, and B. afzelii account for most cases worldwide. A rash may appear at the site of the tick bite, and the patient may experience flu-like symptoms. Left untreated, the patient may suffer from neurologic, arthritic, and cardiac complications.

The syphilis agent Treponema pallidum is most commonly acquired by sexual contact. A skin lesion called a chancre appears at the site of initial contact with the spirochete. T. pallidum later spreads to other sites in the body to cause the flu-like symptoms and rash of secondary syphilis. Once secondary syphilis resolves, the spirochete may persist for years without causing problems. Later, tertiary syphilis can result in damage to vital tissues. Neurosyphilis and cardiovascular syphilis are two common forms of tertiary syphilis.

Leptospira lives in the kidneys of rodents and other reservoir hosts and is shed via urine into the environment. Humans acquire the spirochete by contact of abraded skin or mucous membranes with infectious urine or contaminated water or soil. Leptospirosis patients may initially experience flu-like symptoms. Jaundice and impaired kidney function occur in the potentially deadly form of leptospirosis called Weil's disease.